A communications bus, such as a CAN communications bus in a vehicle, is typically coupled to a plurality of electronic control units (ECUs), which perform various functions and each form a bus node. Thus, an ECU is provided for door control, for example; the closing and/or opening of the doors being able to be controlled and monitored by the ECU. A microcontroller of an ECU, for instance, of a door control system, receives, for example, via the CAN communications bus, a bus message that a window of the doors should be opened. As a result, the microcontroller executes a so-called task (an operation), which triggers a corresponding circuit to open the window.
A frequent case of a fault in motor vehicles is the discharging of the electrical system battery. Although the vehicle is in a rest state (that is, in particular, the ignition is switched off and the vehicle is locked), defective attachment parts in vehicle systems, defective control units (ECUs), flawed software or incompatible hardware states may activate individual control units. Then, activated control units often send messages continually onto the vehicle communications bus (CAN communications bus) and unintentionally discharge the electrical system battery due to a high power consumption. In many cases, mistakenly activated control units are capable of “waking” several other control units connected to the CAN communications bus and/or canceling their “sleep function.” In such a case, the power consumption often discharges an electrical system battery within a few hours and/or when the vehicle stands for a longer period of time.
To date, it has been possible to measure the closed-circuit current of the electrical system battery in a garage, using an ammeter, and to isolate the location of the fault (electrical circuit that is responsible for the increased current flow), by removing the fuses. However, the electrical circuit identified in this manner does not necessarily have to be the fault location triggering the fault. In this context, it may be a sequential fault produced by a malfunction at a different location in the vehicle. Until now, it has not been possible to ascertain, using a diagnostic testing device, the initial fault location for an unwanted activation of a control unit and the accompanying discharge of the battery.